Vinyl chloride resins are excellent in physical properties such as mechanical strength, chemical resistance, and weatherability and yet relatively inexpensive. Semirigid or non-rigid vinyl chloride resins have been widely used as covering materials for automotive interior trim.
Nevertheless, the recent tendency to thickness reduction for cost reduction and requirement for higher performance have boosted the demand for improvements on not only processing stability but also thermal aging characteristics and light resistance during use, and the like.
In order to impart softness to interior trim materials, covering materials of vinyl chloride resins are sometimes lined with urethane. Lining with urethane adversely affects the vinyl chloride resins, especially in terms of resin stability. Improvement for addressing this problem has been awaited.
In recent years, covering materials for crash pads, armrests, headrests, consoles, meter covers, door trim, etc., as automotive interior trim materials, have been expected to have an expensive look with a grain leather pattern, a stitch pattern, etc. as well as lightness in weight and a soft feel.
Conventionally employed covering materials for these applications include vacuum formings of flexible sheets made mainly of a vinyl chloride resin and an ABS resin and rotational castings or slush castings of sol mainly comprising a vinyl chloride resin for paste and a plasticizer (hereinafter referred to as sol castings). The vacuum formings achieve lightness in weight but have a boardy feel lacking softness. It is difficult to produce vacuum formings with an intricate shape such as an expensive-looking grain leather pattern or stitch pattern. In addition, vacuum forming induces a large residual strain so that the vacuum formings easily develop cracks in long-term use.
On the other hand, the sol castings have a soft feel. However, a sol melts fast in a mold because of its low gelling temperature and causes unfavorable phenomena such as flow marks, lipping, and stringing of the sol. Therefore, sol casting has disadvantages, such as poor product surface smoothness, too much time required to drain sol from the mold, and an increased thickness of the covering materials. Furthermore, sol casting involves labor-intensive operation of cleaning a tank, pipes, etc. in case of color changeover. Sol to be cast does not withstand long-term storage due to viscosity change with time.
Powder molding is recently in the spotlight as means addressing the above-described disadvantages and problems. Powder molding techniques typically include fluidization dip coating, electrostatic coating, powder spray coating, and powder rotational or slush molding. Powder rotational molding or powder slush molding is particularly suited to produce covering materials for automotive interior trim.
Powder rotational molding or powder slush molding is a technique in which powder is melted and stuck or sprayed to the inner wall of a rotating or pivoting mold set at a temperature of 180° C. or higher. The mold is integral with a powder feed box, and unmelted powder is automatically or forcibly recovered in the powder feed box.
Systems for heating the mold used in such powder molding include a gas heating oven system, a heating medium oil circulation system, a system of dipping in a heating medium oil or a thermally fluidized sand, and a radiofrequency induction heating system.
A powder composition used in powder molding is required to quickly gel in a mold, to have good releasability from a mold, and not to contaminate a mold as well as to have high heat stability in view of the advantage of powder molding that molding completes at high temperature in short time.
Automotive interior trim materials can be exposed to relatively high temperatures. The temperature of an automotive interior trim is expected to rise near 100° C. in a midsummer sun-heated car. This being the case, automotive interior trim made of vinyl chloride resins will undergo deteriorations (thermal aging), such as coloration.
Where a vinyl chloride resin composition is applied to automotive interior trim, there arises, in addition to the thermal aging problem, a phenomenon that the additives in the composition such as a plasticizer and a stabilizer evaporate into vapors, which adhere to car windows to cause fog. Fog of car windows seriously threatens the safety. Hence, the materials used to make automotive interior trim are also required to have improved fog resistance.
In applications in cold regions like Hokkaido (in winter) or North Europe, automotive interior trim, being exposed in low temperature, becomes hard and brittle and easily breaks on impact. For example, if a material having poor cold resistance is used as a cover of an airbag, the breaking can cause an injury. Materials of automotive interior trim are therefore required to have improved cold resistance.
Automotive interior trim covering materials contain by choice a trimellitic ester plasticizer to improve heat resistance; for a trimellitic ester plasticizer hardly vaporizes when heated in high temperature and hardly migrates into a lining urethane foam layer. However, trimellitic ester plasticizers have a disadvantage of poor cold resistance.
To solve the above problem, the patent document 1 and the patent document 2 propose using a trimellitic ester plasticizer obtained from a specific alcohol component, and the patent document 3 and the patent document 4 propose using a combination of a specific vinyl chloride resin and a trimellitic ester plasticizer. These proposals are still unsatisfactory, however.    Patent document 1: JP-A-2-138355    Patent document 2: JP-A-2-209941    Patent document 3: JP-A-5-279485    Patent document 4: JP-A-10-306187